1. Field of the Invention
The present invention relates to a retrofocus type zoom lens including a negative lens unit located on a magnifying side.
2. Description of the Related Art
In an image projection apparatus such as a liquid crystal projector, an optical system with a wide field angle is required to project a large image at a short distance. In contrast to this, an optical system having a relatively long back focus is required to provide a color combining optical system (such as a prism) between a final surface of a projection optical system and an image display element such as a liquid crystal panel.
An interchangeable lens for a single-lens reflex camera requires a space for providing a mirror box on the rear side (image pickup element side or film surface side) of the interchangeable lens, and thus is required to have a back focus longer than a focal length.
Therefore, as such an optical system, there is used a retrofocus type optical system in which a negative refractive power is arranged on a magnifying side and a positive refractive power is arranged on a reduction side. However, the refractive powers are asymmetrically arranged, and hence it is difficult to correct a distortion or a lateral chromatic aberration.
In recent years the number of pixels of the image display element used in image projection apparatus and the number of pixels of an image pickup element used in single-lens reflex camera have rapidly increased. For that reason, an optical system having high optical performance over the entire magnification range is required. In particular, it is important to correct distortion corresponding to distortion of an image, or a chromatic aberration which influences color blurring or sharpness of an image in a case of a white light source to a high degree.
U.S. Pat. No. 7,292,398 and U.S. Pat. No. 7,480,102 each disclose a structure, in which an optical element having large extraordinary dispersion is provided as a concave lens of a first lens unit (negative lens unit) of the retrofocus type optical system, in which the negative lens unit is provided on the magnifying side, to thereby reduce the lateral chromatic aberration.
In the structure described above, a material having large extraordinary dispersion is influenced by a small negative refractive power to correct a lateral chromatic aberration of the g-line in the vicinity of a wide-angle region. However, when an image obtained by a digital image pickup apparatus is to be greatly enlarged or when the image is to be enlarged and projected onto a screen by the image projection apparatus, it is necessary to further reduce the chromatic aberration. However, in order to correct the chromatic aberration, when the refractive power of an element made of an extraordinary dispersion material is increased to strongly correct the chromatic aberration, the element needs to have a biconcave shape, and hence it is difficult to correct the distortion to a high standard.
Japanese Patent Application Laid-Open No. 2007-178894 discloses a structure in which a concave lens having a partial dispersion characteristic larger than a reference line (θ=−0.001682*νd+0.6438) is provided for the first lens unit (negative lens unit) of the retrofocus type optical system, in which the negative lens unit is provided on the magnifying side.
In the above-mentioned Japanese Patent Application Laid-Open No. 2007-178894, the extraordinary dispersion characteristic and the refractive power of the negative lens are insufficient, and hence a secondary chromatic aberration correction effect is not sufficiently obtained. Therefore, it is difficult to sufficiently correct the lateral chromatic aberration of the g-line in the vicinity of an image at the wide-angle end.